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Astron. Astrophys. 322, 633-645 (1997) 1. IntroductionEver since its discovery, the prominent The interstellar extinction curves along different lines of sight can be reproduced amazingly well by the following simple parameterization (Fitzpatrick & Massa 1986; 1988; 1990) where x is the inverse wavelength (wavenumber in
The main observational constraints concerning the interstellar UV
feature (term with coefficient
Various lines of sight show peculiar extinction curves depending on
the type of environment they pass through. For example, in the
hydrogen poor circumstellar environment of R CrB, the bump is
weaker and shifted to In this paper constraints on the optical properties of the purported interstellar UV feature carrier are derived considering rather general arguments related to chemical composition (as modelled by graphite or Lorentz oscillator models) and clustering (based on direct computations and interpretation using a spectral representation). In Sect. 2 some current theoretical models attempting to explain the characteristics of the interstellar UV feature are discussed. Their main shortcomings are emphasized. In Sect. 3 the profile of the UV feature of graphite is computed for various arrangements of touching spheres. The results are compared qualitatively to the observational constraints. Apart from the peak position falling at the wrong wavelength, the qualitative agreement is found to be good for compact clusters. But variations in chemical composition cannot be included directly in such a model. Therefore, in Sect. 4 a series of single-Lorentz oscillator models are used in conjunction with clustering to investigate the range of parameters (which simulate variations in chemical composition) consistent with the observational constraints. The clustering is modelled via a spectral representation formalism. In Sect. 5 the effects of adding a second Lorentz oscillator to explain the FUV rise are discussed. Interstellar curves along specific lines of sight exhibiting a wide range of FUV curvatures are modelled and plausible (though non-unique) optical constants for the bump grains along these lines of sight are derived. In Sect. 6 physical mechanisms relevant to the expected optical properties of the UV feature carrier are discussed, in particular, dehydrogenation and UV processing. ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 1997 Online publication: June 5, 1998 ![]() |